1. Field of the Invention
The present invention relates to an apparatus and method to enhance lift by cyclically causing the flow over an airfoil to begin to separate from the airfoil and then reattach, and more particularly, to an apparatus and method for increasing the pressure differential created by a rotating impeller.
2. Description of Related Art
Apparatus such as propulsion devices (for example, airplane propellers and marine propulsors), turbomachinery (for example, axial and centrifugal flow compressors), pumps, turbines (axial and centrifugal configurations), and the like, all involve the transfer of energy between a rotating impeller and a working medium fluid. The tools for designing such apparatus have become very sophisticated, using advanced mathematical techniques incorporating complex algorithms requiring the computing capacity of powerful modern computers. But even the most advanced design tools still generally assume steady-state flow conditions. That is, current fluid dynamics design approaches assume that the flow parameters at a given point are constant for any particular set of operating conditions, meaning that at any given location on, say, the blade of a marine propeller operating under a particular set of conditions, the flow parameters do not change over time.
One commentator has put it like this: “In the decades since 1934, engineers and mathematicians have amassed a body of aerodynamic theory sufficient to design Boeing 747s and stealth fighters. As sophisticated as these aircraft may be, their design and function are based on steady-state principles: the flow of air around the wings and the resulting forces generated by that flow are constant over time.” Dickinson, “Solving the Mystery of Insect Flight,” Scientific American, June 2001, pp. 49–57. The same steady-state assumptions are used to design complex propulsion systems such as jet engines. Yet a steady-state fluid dynamics analysis suggests that the seemingly simple way insects propel themselves is actually impossible.
Science has recently come to understand that insect flight in fact involves significant variations over time of the flow field around the insects' wings, caused by complex flapping/rotational wing motion. In other words, understanding insect flight requires non-steady-state analysis. As it turns out, one phenomenon insects take advantage of was observed many years ago. Francis et al., “The Flow Near a Wing Which Starts Suddenly from Rest and then Stalls,” Rep. Memo Aeronautical Research Comm., Aeronautics Laboratory, University of Cambridge, England, Rept. No. 1561, Aug. 8, 1933, shows that a wing that starts at an angle of attack in excess of that associated with steady-state stall travels several chord lengths before experiencing flow separation and loss of lift. An insect uses the delayed stall associated with translational wing motion (in addition to taking advantage of lift and wake capture associated with rotational wing motion) in order to fly. Dickinson et al., “Wing Rotation and the Aerodynamic Basis of Flight,” Science, Vol. 284, Jun. 18, 1999, pp. 1954–60.
The flow over a wing W at the onset of aerodynamic stall is illustrated schematically in FIG. 1. The wing W has a conventional airfoil cross-section, and in normal, level flight the flow stays attached to the top and the bottom of the airfoil. As those skilled in the art understand, when the flow stays attached to the upper and lower airfoil surfaces, the wing generates a lift force L. It is also well known that the magnitude of the lift L is proportional to the airfoil's angle of attack α. This is the angle between the vector representing the airfoil's velocity U∞ through the air and the airfoil chord (a line connecting the leading and trailing edges of the airfoil cross-section). If the angle of attack a increases beyond a critical value, the flow separates from the top surface of the wing W and the lift decreases to a much lower steady stalled value. This is called “stall,” and under normal circumstances it is avoided at all costs.
FIG. 1 illustrates notionally the flow phenomena that occur when an airfoil first enters the flow regime associated with steady-state stall. FIG. 1 shows a wing W traveling from right to left at a high angle of attack α. FIG. 1A shows the wing at time t, just as the wing encounters flow conditions that will lead to steady-state stall. Each of FIGS. 1B to 1E shows the wing position at a very short incremental time τ after the previous figure. As illustrated in FIG. 1, steady-state stall is a process that actually takes a finite time to develop into flow separation from the wing surface. FIG. 1A illustrates that the process of aerodynamic stall begins with a starting vortex C that is generated in the wake of the wing and a vortex CA at the leading edge of the wing W. This vortical flow continues to develop and become more complex as time passes, but as the flow is just beginning to separate from the top surface of the wing, the leading edge vortex CA causes the wing to generate lift as if the flow were still attached to its top surface. In fact, the leading edge vortex CA actually increases the local velocity over the wing, which increases the lift L as illustrated in FIG. 1B. As the wing W continues to travel at an angle of attack α greater than the stall limit, this vortical flow continues to increase in complexity, and the flow eventually does separate from the top surface of the wing, as represented in FIGS. 1D and 1E. It has been suggested that insects can take advantage of this momentary increased lift associated with the beginning of flow separation because they flap their wings and reverse wing direction, causing the flow to reattach before stall actually sets in fully. Dickinson, “Solving the Mystery of Insect Flight” (see above).
Of course, an insect is able to move its wings relative to the air using a complex, periodic flapping and pitching motion that changes the wings' orientation and prevents them from fully stalling. The difficulty in taking the same advantage of this delayed stall mechanism in a manmade device lies in finding a practicable way of introducing the cyclical flow variations necessary repeatedly to approach stall and then permit the working medium flow to reattach as in normal airfoil operation.
Turbomachinery, such as compressors and fans, use rotating blades with an airfoil cross-section to increase the pressure of the working medium. Marine propulsors, such as ships' propellers, torpedo propulsors, and water jets, also use rotating blades with airfoil cross-sections. The amount of energy transferred between any such device and its working medium is a direct result of the amount of lift generated by the blades. Accordingly, any manner of increasing such lift will improve the performance of these devices. However, there is no known mechanism by which such rotating machinery can take advantage of the significant transient lift increases achievable by operating in a delayed stall regime.
Non-steady-state flow leading to delayed stall has been studied. The rotating blades of a helicopter in forward flight experience cyclical variations in angle of attack that can lead to operation in the delayed stall regime for some of the blade travel. For that reason, The Boeing Company, in the course of its helicopter design efforts, has developed and published algorithms for analyzing delayed stall (usually called “dynamic stall” when referring to helicopter rotor blades). Harris et al., “Rotor High Speed Performance, Theory vs. Test,” J. of Amer. Helicopter Soc., Vol. 15, No. 3, April 1970, pp. 35–44; Tarzanin, “Prediction of Control Loads Due to Blade Stall,” J. of Amer. Helicopter Soc., Vol. 17, No. 2, April 1972, pp. 33–46. In particular, a formulation of the Boeing dynamic stall model by Wayne Johnson has proven especially useful for that purpose. Johnson, “Rotorcraft Aerodynamics Models for a Comprehensive Analysis,” Proc. Amer. Helicopter Soc. 54th Annual Forum, Washington, D.C., May 20–22, 9998, pp. 71–93; Nguyen and Johnson, “Evaluation of Dynamic Stall Models with UH-60A Airloads Flight Test Data,” Proc. Amer. Helicopter Soc. 54th Annual Forum, Washington, D.C., May 20–22, 1998, pp. 576–88.
More recently, unsteady flow lift enhancement principles, initially identified and studied in relation to the hydrodynamics of fish, have been explored as concepts that might be incorporated into the control surfaces of small underwater vehicles for the generation of very high maneuvering forces. Preliminary studies have shown that actively controlled flapping control surfaces can generate much higher maneuvering forces than what is possible using steady-state hydrodynamic forces. Bandyopadhyay, “Maneuvering Hydrodynamics of Fish and Small Underwater Vehicles,” Integrative and Comparative Biology, Volume 42, No. 1, February 2002, pp. 102–117.
Further, it has been observed in a modeling study done at the NAVSEA Naval Underwater Weapons Center Division that a reduction in the rotational speed of a marine propulsor can lead to a reduction of radiated noise attributable to various mechanisms, such as blade tonal noise due to wake deficit, trailing edge singing, and ingested turbulence. These noise sources have been shown to be a function of rotational rate to the power of 4, 5, and 6, respectively. Based on a scaling analysis, it was shown that a reduction of 5% in a propulsor's revolutions per minute can reduce noise by 3–5 dB. Bandyopadhyay, et al., “A Biomimetic Propulsor for Active Noise Control: Experiments”, NUWC-NPT Tech. Rept. 11,351, NAVSEA Naval Undersea Warfare Center (NUWC) Division, Newport, R.I., March 2002, pp. 1–15. Accordingly, if it were possible to increase the thrust generated by a particular propulsor, it would be likewise be possible to reduce its rotational speed and thus the noise it generates.
However, in spite of all of the prior art studies and work with algorithms involved in analyzing delayed stall lift enhancement, the fact remains that it has not been utilized in turbomachinery, propulsion devices, and other applications discussed herein.